You have a large number of resources all with a single color background that you want to use, but they are in a custom folder. (Example, Blizz ABS Visual Equipment graphics.) Hence, you can't simply import them. But you don't have photoshop, or maybe there are just way too many to do them one at a time that way. Here's your solution.

Step 1 : Start a new project in RPG Maker. You could use an existing project, but it's easier to keep track of if you use a new projecet.

Step 2 : Import all your resources into your pictures folder. If all the backgrounds are the same, you can select the transparent colors on the first image, and press enter repeatedly through the rest.